Derivative Instruments (Detail) $ in Millions	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Derivative
Number of Hedging instruments | item	0	0
Derivatives not designated as hedging instruments
Derivative
Notional Amount	$ 4,843.1	$ 4,344.7
Derivative Assets	82.4	53.2
Derivative Liabilities	82.0	52.9
Derivatives not designated as hedging instruments | Option contracts
Derivative
Notional Amount		1.9
Derivative Assets		0.4
Derivatives not designated as hedging instruments | Interest-Rate Contracts
Derivative
Notional Amount	4,021.7	3,531.8
Derivative Assets	71.7	45.5
Derivative Liabilities	73.2	46.0
Derivatives not designated as hedging instruments | Interest-Rate Swaps
Derivative
Notional Amount	3,556.1	3,094.5
Derivative Assets	71.5	44.7
Derivative Liabilities	73.0	45.2
Derivatives not designated as hedging instruments | Interest Rate Caps, Floors and Collars
Derivative
Notional Amount	465.6	437.3
Derivative Assets	0.2	0.8
Derivative Liabilities	0.2	0.8
Derivatives not designated as hedging instruments | Spot and forward contracts
Derivative
Notional Amount	815.9	802.7
Derivative Assets	8.5	6.1
Derivative Liabilities	8.8	6.6
Derivatives not designated as hedging instruments | Foreign Exchange Contracts
Derivative
Notional Amount	815.9	808.7
Derivative Assets	8.5	6.4
Derivative Liabilities	8.8	6.9
Derivatives not designated as hedging instruments | Foreign Exchange Contracts | Options purchased | Purchase
Derivative
Notional Amount		3.0
Derivative Assets		0.1
Derivative Liabilities		0.1
Derivatives not designated as hedging instruments | Foreign Exchange Contracts | Options purchased | Sale
Derivative
Notional Amount		3.0
Derivative Assets		0.2
Derivative Liabilities		0.2
Equity Warrants | Derivatives not designated as hedging instruments
Derivative
Notional Amount	5.5	2.3
Derivative Assets	$ 2.2	$ 0.9